This invention relates to a process and apparatus for preparing multi-cellular foamed board of thermoplastic resin.
Various processes and apparatuses have been proposed for preparing a multi-cellular foamed board of thermoplastic resin. These proposals, however, generally are not satisfactory when they are used for preparing a multi-cellular foamed board having a large thickness and a broad width compared with the thickness. If a highly foamed board having a large thickness and broad width is desired having, for example, a thickness of more than 5 mm, a width of more than 20 times the thickness, and furthermore a density of less than 0.3 g/cc, preferably less than 0.1 g/cc, the processes and apparatuses hitherto proposed are inadequate. This is because, if it is tried to prepare by extrusion such a highly foamed board having such a large thickness and broad width, then the resulting board is either unevenly foamed or deformed or disfigured. A board uniformly foamed and having flat surfaces cannot be obtained.
For example, Japanese Patent Publication No. 39-5341 discloses a method for preparing a foamed board, which includes extruding a foamable resin from a die mounted on an extruder to form a flat board, passing the board through a pair of parallel plates to press the board between the plates and to maintain the board in a flat state, and allowing the board to foam freely in the width direction thereof to obtain the foamed board. By this method, however, it is difficult to obtain a foamed board having a broad width compared with a thickness thereof or a highly foamed board. This is because, if this method is used for preparing a broad board or a highly foamed board, the method is liable to give rise to cracks or to cause cells which were formed in the middle portion in the width direction of the board to be ruptured. The resulting board is weak in the middle portion thereof.
Japanese Unexamined Patent Publication No. 54-65771 proposes an extrusion method which includes mounting a specific shaped passage on the forward end of a die. The specific shaped passage is of a fan shape with the forward end protruding its middle portion in the width direction thereof toward the extruding direction and both sides progressively retreating away from the said middle portion. If the specific shaped passage is used, it is difficult to obtain a foamed board having a broad width compared with the thickness thereof or a highly foamed board having a low density. Especially, if this method is used for preparing the highly foamed board, the extruded board is further expanded and deformed while being passed through or after having been passed through the passage, and accordingly a foamed board cannot be obtained which has a shape just as desired.
Japanese Examined Patent Publication No. 43-17825 proposes an extrusion method which includes flattening a resin passage in a die to have a fan shape, thereafter bending the resulting passage perpendicularly to advance the foamable resin in the lateral direction, and extruding the resin through an orifice having the convex forward end. This method, however, cannot provide a foamed board of a large thickness. This is because, if the method is used for preparing a board of large thickness, then the extruded board is curved intricately in the width direction thereof and cannot be a flat board.
Japanese Examined Patent Publication No. 48-20786 proposes an extrusion method using a die having a convex forward end, in which an orifice is opened for extruding a resin. This Publication also proposes that curvature in the convex forward end should be varied according to density of a foamed board to be desired, particularly a radius of the curvature should be decreased when a highly foamed board of a low density is to be desired. Conversely, the radius should be increased when the board of a high density is to be desired. If, however, this method is used for preparing a board having a large thickness, the resulting board is intricately curved in the width direction thereof immediately after having been extruded from the orifice. Thus, a foamed board having an accurate shape as desired cannot be obtained.